There is a great deal of interest at the present time in the development of an engine that can burn coal in a clean and efficient manner. Coal currently costs only about $1.70 per million British Thermal Units (BTU's) compared with diesel fuel at about $5.00 per million BTU's. Moreover, coal is still an abundant natural resource in the United States of America (U.S.) as well as certain other countries and is therefore expected to help satisfy fuel requirements over the next several hundred years. Thus, coal is an attractive fuel because of its relatively lower cost and because it can provide independence from imported petroleum. Another reason for the interest in the use of coal, especially in locomotive engines, is that U.S. railroad companies own a significant amount of coal reserves.
For the above reasons, the U.S. government has been spending a significant amount of money in recent years on the development of gas turbine and reciprocating engines which can use coal as a fuel. Unfortunately, efforts to date in this direction have not been very successful because of either severe engine wear problems caused by the coal fuel and/or because of the bulk and complexity of conventional coal-fueled engine systems.
Basically, there are three ways in which coal has been used to produce power, namely, 1) feeding coal to external combustion engines which generally operate on the Rankine cycle, 2) injection of coal dust or slurry into a gas turbine engine or diesel-cycle internal combustion engine or, alternatively, ingestion of coal dust into a diesel-cycle engine's intake air, and 3) gasification of the coal and use of the coal gas as a fuel for a modified diesel-cycle engine, gas engine, or gas turbine engine.
The use of coal in an external combustion engine which operates on the Rankine cycle dates back to the beginning of the industrial revolution which occurred in the late eighteenth century. This type of powerplant, in which coal is burned to create steam to operate a reciprocating engine, was extensively used in both mobile and stationary applications. Unfortunately, while a steam powerplant of this type can be used in vehicles, its maximum efficiency is limited to about 15%. Very large Rankine-cycle powerplants having steam turbines are widely used today for the generation of electrical power. While such powerplants have reasonable efficiency for that application, they are not suitable for mobile applications.
The use of a coal slurry as a fuel for diesel-cycle engines is shown, for example, in U.S. Pat. No. 4,558,664 issued to Robben on Dec. 17, 1985 and U.S. Pat. No. 4,782,794 issued to Hsu et al. on Nov. 8, 1988. The use of fumigated coal dust as a fuel for diesel engines is shown, for example, in U.S. Pat. No. 4,056,080 issued to Rutz et al. on Nov. 1, 1977. Neither of these methods have produced much success. While an engine will operate on these fuels with reasonable efficiency, especially at lower speeds, the coal particles and ash cause combustion cylinder wear which is about 50 to 100 times greater than that experienced with diesel fuel. In addition, wear and other problems associated with the fuel system appear to be very difficult to overcome. Consequently, it is not felt that these approaches have a very good long-range potential.
Powerplants which combine a coal gasifier with a reciprocating Otto-cycle engine or gas turbine engine have been known for many years. In the case of a powerplant utilizing a gas turbine engine, coal has been gasified at about 20 atmospheres pressure. After it exits the gasifier, the coal gas is cleaned by various means such as filters for removing particulate matter and zinc ferrite pellet beds for removing sulfur while the temperature of the coal gas is still near the temperature at which it exited the gasifier. The hot cleaned gas, still at about 20 atmospheres pressure, is then injected into the combustor of the gas turbine engine. Operation of the gasifier at the elevated pressure of 20 atmospheres ensures that the coal gas is at a greater pressure than exists in the combustor of the gas turbine engine when injection occurs. This arrangement also reduces the size of the gasifier because of the higher density air supplied thereto and much of the energy contained in the hot coal gas which exits the gasifier can be recovered by expansion through the gas turbine engine. In addition, injection of the coal gas while still at elevated temperature prevents condensation of tars in the coal gas, which can cause serious contamination of various working components of the powerplant.
While manufactured coal gas can be effectively utilized in a gas turbine engine, this type of engine has limitations especially when used in relatively small sizes. Principal among these is lower than desired efficiency especially at reduced power output which occurs in a significant portion of powerplants used for mobile applications. Consequently, for mobile applications it is considered to be more desirable to use the producer gas to fuel a reciprocating engine.
The use of a gasifier to produce a low BTU coal gas to fuel a companion reciprocating engine can provide reasonable efficiency. While the energy content of coal gas produced is only about 100 to 200 BTU's per cubic foot (compared with natural gas whose energy content is about 1000 BTU's per cubic foot), this is not considered a serious problem if the coal gas is utilized immediately following its production in the gasifier. In addition, the coal gas can be cleaned of particulate matter and sulfur which naturally exist in the coal thus assuring adequate life and environmental acceptability for the exhaust products emitted from the engine.
To date, such powerplants have gasified the coal at relatively low pressures of about 1 atmosphere. After exiting the gasifier, the coal gas is cooled to near ambient temperature to condense and remove the tars. The particulates and sulfur in the coal gas are then removed by filtration and chemical reactions. The cooled and cleaned coal gas is then directed to the air intake system of a spark-ignited Otto-cycle or dual-fuel (i.e., ignited by pilot diesel fuel) gas engine.
Engines of this type which induct the fuel gas and intake air as a mixture during an intake phase of the engine cycle are susceptible to detonation or knock which can cause failure of the engine. In order to prevent these problems, these engines are compromised by deliberately limiting their power output and/or by operating them at relatively low compression ratios. The efficiency of the spark-ignited Otto-cycle engine is inferior to that of a diesel-cycle engine not only because of the spark-ignited Otto-cycle engines' relatively lower compression ratios but also because they inherently experience throttling losses while operating at reduced power output.
The loss of available energy which occurs with the cooling of the producer fuel gas prior to cleaning and induction into the engine as well as the present use of engines which are less efficient than diesel-cycle engines result in a less-than-desired overall efficiency for this type of powerplant. In addition, the presently large size of the gasifier (which operates at ambient or low pressures) and associated heat exchangers make this type of powerplant too large and unattractive for use in mobile applications when compared with conventional diesel-cycle engines currently used. Furthermore, conventional gasification of fossil fuels, particularly coal, produces not only fuel gas but also particulates mixed therein which can cause wear and sealing problems in various moving parts of the gasifier.
The present invention is directed to overcoming one or more of the problems as set forth above.